It has long been known that single-walled cups do not present a very good thermal barrier to insulate the fingers of a person holding such a cup containing a hot beverage from the heat given out by that beverage.
Better heat insulation is obtained by using a so-called "double-walled" cup which is, in fact, two cups, one being inside the other, joined at their rims and having an air space between the outer wall of the inner cup and the inner wall of the outer cup. This wall/air/wall barrier has proved effective in minimizing the amount of heat from a hot beverage inside the inner cup which will reach the fingers of a person holding the cup, thereby enabling the cup to be held without discomfort.
Such double-walled cups are, however, expensive to produce in that two single-walled cups have to be made and then joined together. Furthermore, a large part of the outer of the two cups is not always put to good use. A cup can readily be held in the fingers without the fingers going below half-way down the wall of the cup, and indeed it is possible, although difficult, to hold a cup without the fingers going below about 10 mm from the rim of the cup.
United Kingdom Patent Specification No. 1,379,371 describes an injection moulded cup which has a substantially cylindrical upper portion forming the mouth of the cup and lower portion tapering towards its bottom. A substantially cylindrical collar is connected to the transition between these two portions by means of a web. This arrangement allows the user to grip the cup by means of the collar and the provision of the air gap between the collar and the container wall means that heat is only transmitted to the user's fingers by conduction along the web.
The complicated nature of the shape of the cup shown in United Kingdom Pat. No. 1,379,371 makes it, however, difficult to manufacture and the only really feasible method of manufacture is by the expensive process of injection moulding. The vertical orientation of the collar means the collar and web construction has to have considerable rigidity, and therefore thickness, in order to prevent the collar deforming when gripped, to such an extent that the collar engages the wall of the cup. Further the design of the cup is such that a large amount of material is needed to form the cup.
Various other constructions have been developed for cups intended to provide heat insulation such as those described in U.S. Pat. No. 3,612,346, United Kingdom Pat. No. 1,325,230 and Belgian Pat. Nos. 703,364 and 705,397. Although these cups and those described above have been designed for providing thermal insulation by an air space between the wall of the cup in contact with the hot liquid and the cup gripping surface, their construction apparently requires considerable quantities of material (thickness) to insure wall rigidity for maintaining the essential air space when the cup, heavy with hot liquid, is tightly gripped by hand.
It will be appreciated that vending cups are disposable items and that relatively small difference in cost of a single item can make the difference between a cup being commercially usable or being a mere impractical concept.